Add Gateway Control Center with service control and pairing management

New Gateway section in the Manage group:
- Service controls: Start/Stop/Restart buttons calling hermes gateway CLI
- Status display: state (running/stopped), PID, loaded indicator, stale
  service warning, exit reason, last update timestamp
- Platform cards: each connected messaging platform with connection state
  (reads from gateway_state.json)
- Pairing management: approved users list with revoke button, pending
  pairing codes with approve button
- Auto-refreshes via HermesFileWatcher when gateway state changes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scarf/scarf/Navigation/AppCoordinator.swift

@@ -9,6 +9,7 @@ enum SidebarSection: String, CaseIterable, Identifiable {
     case memory = "Memory"
     case skills = "Skills"
     case tools = "Tools"
+    case gateway = "Gateway"
     case cron = "Cron"
     case logs = "Logs"
     case settings = "Settings"
@@ -25,6 +26,7 @@ enum SidebarSection: String, CaseIterable, Identifiable {
         case .memory: return "brain"
         case .skills: return "lightbulb"
         case .tools: return "wrench.and.screwdriver"
+        case .gateway: return "antenna.radiowaves.left.and.right"
         case .cron: return "clock.arrow.2.circlepath"
         case .logs: return "doc.text"
         case .settings: return "gearshape"